1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a method for efficiently specifying a mapping relationship between transport blocks and codewords, and a method for efficiently transmitting a downlink signal using the same.
2. Discussion of the Related Art
Error control algorithms, which are currently used in communication systems, may be broadly classified into an automatic repeat request (ARQ) scheme and a forward error correction (FEC) scheme. The ARQ scheme includes stop-and-wait ARQ, go-back-N ARQ, selective repeat ARQ, etc. Stop-and-wait ARQ refers to a scheme for confirming whether a transmitted frame has been accurately received. And after confirming accurate reception of the previous frame, the transmitting side transmits the next frame. Go-back-N ARQ refers to a scheme for transmitting N successive data frames, and if transmission is not successful, all data frames followed by an error-generated frame are re-transmitted. Selective repeat ARQ refers to a scheme for selectively re-transmitting only error-generated frames.
Meanwhile, hybrid automatic repeat request (HARQ) refers to an error control scheme combining re-transmission and error correction that maximizes error correction coding capabilities of data received during re-transmission. HARQ may be divided into chase combining (CC) HARQ and incremental redundancy (IR) HARQ according to characteristics of transmitted bits during re-transmission. CC HARQ obtains a gain by raising a signal-to-noise ratio (SNR) in a receiving stage using data used for previous transmission in re-transmission. IR HARQ gains a coding gain in a receiving stage by combining redundancy bits during re-transmission to improve performance.
FIG. 1 is a view illustrating the concept of a stop-and-wait ARQ scheme in HARQ.
In a stop-and-wait ARQ protocol, a determination as to whether to re-transmit data is made by individually receiving an acknowledgement (ACK)/negative acknowledgement (NACK) signal from a receiving stage after one process block is transmitted. Although the stop-and-wait ARQ scheme is the simplest and most efficient transmission method, link transmission efficiency is lowered by a round trip time (RTT) until a transmitting stage receives the ACK/NACK signal from the receiving stage.
FIG. 2 is a view illustrating an N-channel stop-and-wait HARQ protocol scheme.
The N-channel stop-and wait HARQ protocol scheme performs N independent stop-and-wait HARQ operations simultaneously, in order to overcome a shortcoming described with reference to FIG. 1. The N-channel stop-and-wait HARQ protocol scheme can reduce a processing delay.
Meanwhile, a multiple-input multiple-output (MIMO) scheme increases system capacity by simultaneously transmitting multiple data streams spatially using two or more transmit/receive antennas in a base station and a mobile terminal. The MIMO scheme can obtain a transmit diversity gain or a beamforming gain using multiple transmit antennas.
A transmit diversity scheme transmits the same data through multiple transmit antennas for reliable data transmission in fast time-varying channel environments and has an advantage in the case when the channel-related feedback information is not received from a receiving end. A beamforming scheme is used to increase a signal to interference plus noise ratio (SINR) of a receiver by multiplying weighting values for the multiple transmit antennas. In general, since a frequency division duplex (FDD) system has independent uplink and downlink channels, high reliability channel information is required to obtain a proper beamforming gain and therefore additional feedback information received from the receiver is used.
A spatial multiplexing scheme for a single user and for multiple users will be described in brief.
FIG. 3 is a view illustrating the concept of spatial multiplexing and spatial divisional multiple access schemes used in a MIMO communication system.
Spatial multiplexing for a single user is called SM or single user (SU)-MIMO and transmits data using a plurality of antennas for one user as shown in the left side of FIG. 3. Therefore, the capacity of a MIMO channel increases in proportion to the number of antennas. Meanwhile, spatial multiplexing for multiple users is called spatial division multiple access (SDMA) or multi-user (MU)-MIMO and transmits and receives data through a plurality of user antennas as shown in the right side of FIG. 3.
A MIMO scheme includes a single codeword (SCW) method which simultaneously transmits N data streams using one channel encoding block and a multiple codeword (MCW) method which transmits N data streams using M (where M is equal to or less than N) channel encoding blocks. Each channel encoding block generates independent codewords and each codeword is designed to be able to independently detect errors.
On the other hand, the above-described codeword is transmitted through one or more layers and information transmitted through codewords may be swapped for transmission. In a wireless communication system which can simultaneously transmit a plurality of codewords, the size of a specific codeword itself or information transmitted through the specific codeword may be 0 in some cases.